Chitin, a linear amino polysaccharide composed of β-(1→4)-linked 2-acetamido-2-deoxy-β-D-glucose units found in the outer skeleton of arthropods, is the second most plentiful natural polymer after cellulose (Bartlett et al., Science, 310: 1775-1777 (2005)). Its bioactivity, biocompatibility, and low toxicity make it suitable for controlled drug release formulations, cosmetics, food preservation, fertilizers, or biodegradable packaging materials, while its ability to absorb both metal ions and hydrophobic organic compounds make it useful in waste water processing and other industrial applications (Synowiecki et al., Crit. Rev. Food Sci. Nutr., 43:145-171 (2003) and Kumar, React. Funct. Polym., 46:1-27 (2000)). However, due to its high density of hydrogen bonds, chitin is completely insoluble in water, most organic solvents, dilute acidic solutions, and dilute basic solutions. Thus, various chemical modifications have been applied to make chitin more easily soluble, including N-deacetylation to form chitosan (Sashiwa et al., Carbohydr. Polym., 39:127-138 (1999)).
Chitin can be obtained commercially in pure grade or practical grade (PG-chitin). PG-chitin is primarily produced from crustacean shells by a chemical method that involves acid demineralization of the shell, followed by removal of shell proteins by alkali treatment, and then decolorization (Percot et al., Biomacromolecules, 4:12-18 (2003)). It can be further purified by methanesulfonic acid treatment to obtain pure chitin (Hirano and Nagao, Agric. Biol. Chem., 52:2111-2112 (1988)). But even though the current industrialized chemical process isolates chitin efficiently, the chitin molecular weight (MW) is reduced during processing (Synowiecki et al., Crit. Rev. Food Sci. Nutr., 43:145-171 (2003)). A less chemical- and energy intensive process for obtaining the chitin, and a purer, higher molecular weight chitin product is desirable for many applications including fiber spinning.
Chitin is known to form microfibrillar arrangements in living organisms, and the presence of microfibrils suggests that chitin should be a good candidate for fiber spinning (see U.S. Pat. No. 3,892,731). However, only a few papers describing the spinning of chitin fibers have been reported, mainly due to the limited number of solvent systems which can readily dissolve chitin in sufficient quantity and with appropriate rheology for spinning Thus, producing chitin fibers or even films continues to be a challenge in chitin research.
In most cases where chitin fibers have been produced, commercial chitin powder has been used with solvent systems such as (1) halogenated solvents (e.g., trichloroacetic acid (TCA), dichloroacetic acid (DCA), or formic acid-DCA mixtures, or (2) amide-LiCl systems (e.g., N,N-dimethylacetamide (DMAc)-5% LiCl) (see U.S. Pat. No. 3,892,731; Tokura et al., Polym. J., 11:781-786 (1979); and Rutherford and Austin in Proceedings of the First International Conference on Chitin/Chitosan, ed. Muzzarelli and Parises, MIT Sea Grant Report, MIT SG 78-7, 1978, pp. 182-192). The drawbacks of these methods include the use of corrosive chemicals that can degrade the polymer upon even short exposures and difficulties in the complete removal and recovery of the solvent from the fiber. An environmentally-benign solvent that could readily solubilize chitin or even crustacean shells without derivatization would be greatly beneficial in this arena.
Crustacean shells (e.g., shrimp shells) contain not only chitin, but also large amounts of protein, mineral salts, and a small amount of lipids. Thus, crustacean shells are even harder to dissolve than either PG-chitin or pure (native) chitin. Thus, what are still needed are new solvents and methods for forming fibers and other structures from chitin obtained directly from a chitinous biomass such as crustacean shells. The subject matter disclosed herein addresses these and other needs.